It's pretty well normal wear for tires that have an "island" tread. The "islands" rock forward and back under acceleration and braking.
If you look at car tires, especially narrow ones with the same "island" pattern, you'll see the same wear. My old tire shop guy called it "Heel and Toe" wear. Others call it feathering, cupping, scalloping, whatever.
Just change the tread pattern design on the next set, or get a non-metal belted tire that will flex a bit more. A slick won't exhibit the wear pattern if you think about it.